Hydrogenated sugar (also referred to as “sugar alcohol”) means a compound obtained by adding hydrogen to the reductive end group in sugar, and generally has a chemical formula of HOCH2(CHOH)nCH2OH wherein n is an integer of 2 to 5. According to the carbon number, hydrogenated sugar is classified into tetritol, pentitol, hexitol and heptitol (4, 5, 6 and 7 carbons, respectively). Among them, hexitol having 6 carbons includes sorbitol, mannitol, iditol, galactitol, etc. and in particular, sorbitol and mannitol are very useful materials.
Anhydrosugar alcohol has a diol form with two hydroxyl groups in the molecule, and can be produced by using hexitol derived from starch (for example, Korean Patent No. 10-1079518 and Korean Laid-open Patent Publication No. 10-2012-0066904). Because anhydrosugar alcohol is an environmentally friendly material derived from recyclable natural resources, it has received much interest for a long time and researches on its production continue to proceed. Among such anhydrosugar alcohols, isosorbide produced from sorbitol has the widest industrial applicability at present.
Anhydrosugar alcohol can be used in various fields including treatment of heart and blood vessel diseases, medicaments such as patch adhesive, mouthwash, etc., solvents for compositions in the cosmetics industry, emulsifiers in the food industry, etc. In addition, it can increase the glass transition temperature of polymer materials like polyester, PET, polycarbonate, polyurethane, epoxy resin, etc., and improve the strength of such materials. Furthermore, because anhydrosugar alcohol is an environmentally friendly material derived from natural resources, it is very useful in the plastics industry such as bioplastics and the like. It is also known that anhydrosugar alcohol can be used as an adhesive, environmentally friendly plasticizer, biodegradable polymer, and environmentally friendly solvent for water-soluble lacquer.
As such, anhydrosugar alcohol is receiving much interest because of its wide applicability, and the level of practical industrial application thereof is increasing. However, the conventional methods of producing anhydrosugar alcohol have limitations of high cost for the catalyst used in the dehydration reaction, low conversion rate, and low yields of distillation and purification, etc.
In order to produce anhydrosugar alcohol economically, it is essential to employ a technology of distilling anhydrosugar alcohol from the resulting liquid of conversion reaction within a short time with high yield and high purity.
As a distillation technology of distilling the conversion liquid after dehydration reaction, batch distillation or simple distillation—wherein anhydrosugar alcohol is simply distilled under reduced pressure directly after the conversion reaction in the reactor—is known.
By batch distillation or simple distillation, however, commercial-scale economical production is difficult since the distillation time is long. In addition, if the resulting liquid of a conversion reaction is distilled at a low temperature (e.g., 170° C. or lower), the distillation time increases, and if distilled at a relatively high temperature (e.g., 170° C. or higher), the distillation time decreases but the anhydrosugar alcohol is thermally decomposed at 170° C. or higher, and byproducts such as formic acid, furfural, etc. are generated, by which the purity of the product and the pH of the distillate are lowered. That is, as compared with the wiped-film evaporation explained below, since batch distillation or simple distillation requires relatively longer retention time of distillate and higher distillation temperature, thermal decomposition of alcohol is induced, generating the problem of lowering purity and yield of the distillate. To prevent such a thermal decomposition, use of an additive is required.
In order to overcome the deficiencies of batch distillation or simple distillation in distilling anhydrosugar alcohol from the resulting liquid of a conversion reaction, U.S. Pat. No. 7,439,352 suggested a technology of distilling anhydrosugar alcohol by wiped-film evaporation using an external condenser. In the wiped-film evaporation technology disclosed in this US patent, the condenser is operated outside of the distillator. In this type, however, the maximum vacuum circumstance that can be formed in the distillator technically is 1 mmHg; under such a vacuum degree the distillation temperature should be 170° C. or higher in order to conduct the distillation effectively. However, as stated earlier the anhydrosugar alcohol such as isosorbide is thermally decomposed at a distillation temperature of 170° C. or higher, and thereby the distillation yield and distillation purity are lowered. Accordingly, in the above US patent, the purity of the single-step distillation product is 97.1% or the like and the distillation yield is 80% or the like. However, such levels of purity and yield are still not suitable for a commercial-scale mass-production process.
In the above US patent, the isosorbide purity is increased to 99.9% through distillation of two (2) stages. However, the overall distillation yield is low (first stage/second stage yield 77.5%). To resolve the problem of such a low overall distillation yield, the above US patent employs a separate process for crystallizing the residue of the two-stage distillation and recovering isosorbide therefrom. However, in the overall process flow as such, the isosorbide is produced by two separate processes consequentially, which makes the uniform quality control much more difficult than a single-process production.
Therefore, a technology of producing anhydrosugar alcohol, which can provide high purity (e.g., 98% or higher) and a content of sorbitol and sorbitan isomer as impurities of less than 0.1% with a high overall distillation yield (e.g., 94% or higher, more preferably, 95% or higher), is required.